Re: [RFC PATCH] math.h: Account for 64-bit division on i386

[Guenter Roeck <linux@roeck-us.net>]

On Sat, Apr 19, 2025 at 01:51:46PM +0200, cem@kernel.org wrote:
> From: Carlos Maiolino <cem@kernel.org>
> 
> Building linux on i386 might fail if a 64bit type is passed to

i386 actually builds. Its compiler is probably able to convert
the offending mult_frac() without helpers since the divisor is
a constant. I see the problem with openrisc and parisc, with
gcc 13.3.0.

> mult_fract(). To prevent the failure, use do_div() for the division
> calculation instead of hardcoding a / b.
> 
> Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202504181233.F7D9Atra-lkp@intel.com/
> ---
> 
> I'm sending it as a RFC because I didn't to extensive testing on this
> patch, also I'm not sure if mult_frac() was intended to work on 32-bit
> only types. If that's the case, perhaps, a new mult_frac64() might be a
> better idea?!
> 
>  include/linux/math.h | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/math.h b/include/linux/math.h
> index 0198c92cbe3e..05ea853b75b4 100644
> --- a/include/linux/math.h
> +++ b/include/linux/math.h
> @@ -133,15 +133,16 @@ __STRUCT_FRACT(u32)
>  #undef __STRUCT_FRACT
>  
>  /* Calculate "x * n / d" without unnecessary overflow or loss of precision. */
> -#define mult_frac(x, n, d)	\
> -({				\
> -	typeof(x) x_ = (x);	\
> -	typeof(n) n_ = (n);	\
> -	typeof(d) d_ = (d);	\
> -				\
> -	typeof(x_) q = x_ / d_;	\
> -	typeof(x_) r = x_ % d_;	\
> -	q * n_ + r * n_ / d_;	\
> +#define mult_frac(x, n, d)		\
> +({					\
> +	typeof(x) x_ = (x);		\
> +	typeof(n) n_ = (n);		\
> +	typeof(d) d_ = (d);		\
> +					\
> +	typeof(x_) r = do_div(x_, d_);	\
> +	r *= n_;			\
> +	do_div(r, d_);			\
> +	x_ * n_ + r;			\
>  })
>  

Unfortunately that doesn't work. I get build errors on parisc.

In file included from ./arch/parisc/include/generated/asm/div64.h:1,
                 from ./include/linux/math.h:6,
                 from ./include/linux/kernel.h:27,
                 from ./arch/parisc/include/asm/bug.h:5,
                 from ./include/linux/bug.h:5,
                 from ./include/linux/mmdebug.h:5,
                 from ./include/linux/mm.h:6,
                 from mm/page_alloc.c:19:
mm/page_alloc.c: In function 'boost_watermark':
./include/asm-generic/div64.h:183:35: error: comparison of distinct pointer types lacks a cast [-Werror]
  183 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
      |                                   ^~
./include/linux/math.h:142:24: note: in expansion of macro 'do_div'
  142 |         typeof(x_) r = do_div(x_, d_);  \
      |                        ^~~~~~
mm/page_alloc.c:2010:21: note: in expansion of macro 'mult_frac'
 2010 |         max_boost = mult_frac(zone->_watermark[WMARK_HIGH],

That is just one example. It seems to affect all uses of
mult_frac().

Guenter